DAOS-16639 object: fix assertion

[wangdi1]

Invalid hole extent might be left by process_hole_ult(), so let's skip it.

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[github-actions]

Ticket title is 'agg_diff_preprocess() Assertion '!extent->ae_hole' failed'
Status is 'Open'
Labels: 'scrubbed_2.8'
https://daosio.atlassian.net/browse/DAOS-16639

[wangdi1]

close this one for fixing the branch name, and moved to #15329